In recent years, the electroless plating of plastic parts and printed circuit boards has become a very common manufacturing process, typically involving a series of closely controlled processing steps. The actual step of plating from an electroless bath is typically carried out in a plating tank that is lined with plastic or stainless steel to make it non-catalytic to the plating solution. Inasmuch as most electroless baths are operated at elevated temperatures, the tank normally is equipped with means for heating the solution. An electroless plating bath requires frequent adjustment for consistent performance. Agitation of the bath is essential to insure renewal of the plating solution in contact with the surface being plated. Careful cleaning and conditioning of the parts to be plated is also very essential to avoid misplate and unevenness in the thickness of the plate. The bath is filtered continuously or periodically to remove impurities and metal particles that may seed out from solution.
The plating bath requires replenishment of the solution, typically by the addition of a mixture of correctly apportioned ingredients, including the metal salt, reducing agent, stabilizers and brighteners. This may require extensive monitoring of the bath, accompanied by the manual addition of the chemicals to the bath. Various automatic regulators have been developed, but these typically use test equipment which measures the pH of the bath or the concentration of metal ions in the bath and makes adjustments based on these measurements. Normally the testing involves removing a small portion of the bath from the plating tank, measuring the pH or metal concentration in the removed portion perhaps preceded by coating, diluting, buffing, etc., and then returning the test sample to the bath.
In the recent past, one method of monitoring electroless copper baths has been to use an in-tank optical sensor to detect changes in the transmissibility of the plating bath. As the copper ions in a bath become depleted, the bath becomes less opaque resulting in an increase in the amount of light that passes between a light source and a detector, spaced a fixed distance apart. The change in light detection can be used to activate a pump or other device to transfer addition agents into the plating tank. Because electroless copper plating is normally carried out at a temperature of 140.degree. F. or below, such a monitoring device is relatively simple.
On the other hand, other electroless plating baths such as chromium, cobalt, gold, iron and nickel are run at temperatures between 170.degree. F and 205.degree. F., accompanied by the high degree of agitation and the rapid evolution of hydrogen gas at these elevated temperatures. The in-tank probes used to monitor copper electroless plating baths have been found to be totally inadequate for controlling the addition of metals salts, reducing agents and pH adjusters in these heated plating baths. U.S. Pat. No. 4,096,301 (Re 31,694) describes one method of controlling an electroless copper bath by withdrawing a sample of the bath at a constant rate and subjecting the sample to three separate sequential analyses to test for pH and for concentrations of copper and formaldehyde. Fresh solutions are metered into the bath to adjust for pH and to replenish the depleted chemicals. U.S. Pat. No. 4,479,852 describes a method of monitoring organic additives in an acid copper plating bath. The method involves plating onto a cathode at constant current density and voltage, followed by immersing the plated cathode into the bath along with a reference electrode and an anode, passing a current at a constant current density from the anode to the cathode, and measuring the voltage difference between the reference electrode and the cathode. The measured voltage differences are then compared with previously determined values of known concentrations.